For the first time in up to five million years, the concentration of carbon dioxide in our atmosphere is about to reach 400 parts per million (ppm). Former NASA scientist James Hansen warned that levels over 350ppm would destabilize the earth’s climate, but now we have far exceeded that figure with a record-breaking weekly average of 398.5ppm recorded on Monday, The Guardian reports. Researchers at the Earth Systems Research Laboratory in Hawaii expect we will hit the 400ppm milestone in May, 2013.

“I wish it weren’t true but it looks like the world is going to blow through the 400ppm level without losing a beat. At this pace we’ll hit 450ppm within a few decades,” Ralph Keeling, a geologist with the Scripps Institution of Oceanography which operates the Hawaiian observatory, told The Guardian.

Tim Lueker, an oceanographer and carbon cycle researcher with Scripps CO2 Group, said that this “sobering milestone” should be a wake up call for governments to support clean energy and slash emissions.